As is well known in this specific technical field, there has for some time been a requirement to make the management of public street lighting installations more efficient and more economical. Satisfying this requirement would make it possible to extend the technical solutions which might be adopted for this purpose to other fields of application, for example in the public and industrial context.
Still more in particular, there is currently urgent demand from many public authorities so that they can pursue major economies in the consumption of electrical energy used for street lighting or lighting public areas.
If the problems underlying the present invention are to be better understood, it is first of all necessary to explain some concepts and notions relating to gas discharge lamps, for example xenon discharge lamps or a high pressure sodium discharge lamps.
Studies carried out on xenon (Xe) discharge lamps, used in “climatic chambers”, namely in chambers in which accelerated ageing conditions are simulated by reproducing all possible climatic conditions (generally humidity, acidity, temperature and pressure), have revealed interesting information relating to current flow modes in the lamp bulbs.
These bulbs are usually made from quartz and filled with various types of gas which are capable of supporting the flow of electric current once they have been brought to a particular state of excitation, known as ionisation, in the switch-on phase. In practice, the gas molecules, once electrically charged (preionisation phase), undergo a change in their electrical state, changing from an insulator to a conductor. In this latter state, they can thus permit the flow of electric current in exchange for strong light emission which usually arises due to a thermoelectric effect.
The current may thus be externally controlled by means of appropriate electrical driving circuits, with the aim of regulating steady-state light emission, in terms of both wavelength (spectrum) and intensity (lumens per m2—i.e. lux—or W per m2, depending on the reference variable stated in the regulations).
Further studies carried out by the Applicant have focused on other types of gas discharge lamps to investigate whether it might be possible to apply the knowledge and experience gained with xenon discharge lamps. Such further studies primarily related to high pressure sodium (HPS) lamps, which are today those which achieve the best ratio between luminosity (lumens emitted) and current consumed.
This type of lamp is also used very widely in public lighting installations thanks to the relative structural simplicity required by the installation. The currently most widespread lamps in fact comprise lamp posts with a lamp 2 connected in series to a current-limiting coil 3 and to an appropriate ignitor 4 which functions as a starter for the gas preionisation phase; this arrangement is connected to the general 230 V AC electrical supply network, as shown schematically in FIG. 1.
The lamps are generally rated at 150 W, although there are installations of up to 2000 W.
It should also be noted that HPS sodium lamps behave overall very differently from xenon lamps.
The emitted light spectrum is in fact very different, sodium lamps virtually completely lacking the ultraviolet component and instead comprising a very strong yellow/orange component which also characterises their appearance in the streets or squares where they are usually installed.
The studies carried out by the Applicant have made it possible to investigate some characteristics which are listed below, for each of which a brief general evaluation is provided:                electrical efficiency achievable with conventional installations ranges from a minimum of 60% to a maximum of 79%. This calculation is made by straightforwardly applying the ratio “output power”/“input power”. The variation in efficiency obtained is determined by the lamps from the various manufacturers, each of which suggests current-limiting coils of a value which varies from manufacturer to manufacturer;        the power factor of the installation is quite low (0.53-0.65) and in any event outside any existing regulations (>=0.85). If it were desired to use a power factor correction capacitor, the problem of still lower electrical efficiency would have to be addressed;        the operating temperature of the coils allows for increases of even greater than 60° C. above ambient temperature;        it has been found that there is always a small continuous component in the current passing through the lamp, both immediately after switch-on (i.e. when the gas is yet to achieve thermal equilibrium), and after some minutes of operation. This fact brings about an appreciable degradation in both electrical efficiency and the average life of the lamps: in the first case because there is a constant power loss due to heating which cannot be used for conversion into radiant energy, in the second case because there is a constant migration of material from the internal electrodes of the lamp resulting in premature consumption. It is also worthwhile noting that gas excitation always proceeds by means of a high intensity electric field and that this makes it possible to “detach” some atoms from the material constituting the lamp electrodes resulting their actually being eroded and, ultimately, physically consumed;        it is never possible to switch the lamps back on without having allowed a cooling time which may in some cases extend to several minutes;        it is not possible to switch the lamps on if the supply voltage is less than 195 V AC; in contrast, it is possible to keep them switched on at voltages of as low as approx. 180 V AC;        if the supply voltage is raised to approx. 250 V AC, electrical efficiency drops fairly quickly and assumes an average value of 65%.        
The technical problem underlying the present invention is that of devising a command and control device for discharge lamps, for example high pressure sodium lamps, which has structural and functional characteristics such as to permit only active power to be drawn from the electrical network; in this manner, the efficiency of said electrical distribution network would be maximised.
Another object of the invention is to conceive a command and control device for discharge lamps which has structural and functional characteristics such as to permit the lamp to be driven at high frequency by controlling and regulating the voltage and current drawn and cutting any continuous current component absolutely to zero with the aim of maximising the average life of said lamp.
A further object of the invention is to be able to regulate luminous flux at will, as a function of the state of ageing of the lamp, so specifically enabling programmable light emission.
Another object of the invention is to permit the lamp also to be switched on at voltages below those which are currently possible, eliminating the waiting time for immediately switching back on and so actually allowing it to be switched back on when hot (hot restart).
Finally, an object of the present invention is, not least, to maximise the electrical efficiency of the command and control device for discharge lamps, also minimising operating temperatures and structural dimensions, with an aim of above all reducing the quantity of electrical energy necessary for achieving the radiation specified in international regulations.